Polyester diverting agents for low-temperature oil wells

ABSTRACT

A hydraulic fracturing method for recovering oil from a low-temperature subterranean oil formation is disclosed. Before, during, or after inducing hydraulic fracturing within the formation, a particulate, degradable polyester diverting agent is introduced into the formation in an amount effective to improve oil production from the formation. The diverting agent is allowed to degrade, and oil is recovered. The diverting agent has a melting point greater than the average temperature of the formation and is selected from: (i) ethylene glycol succinates; (ii) acid-terminated ethylene glycol succinates; (iii) acid-terminated polyglycolic acids; (iv) acid-terminated polylactic acids; (v) mixtures of any of (i) through (iv) with a half acid ester; and (vi) mixtures of polylactic acids or polyglycolic acids with a half acid ester. These easily synthesized classes of particulate polyester diverting agents degrade more effectively than polylactides under the conditions present in low-temperature wells and should help to enhance well productivity.

FIELD OF THE INVENTION

The invention relates to polyester compositions and their use inhydraulic fracturing as diverting agents, particularly forlow-temperature oil wells.

BACKGROUND OF THE INVENTION

Hydraulic fracturing is an oil-and-gas well stimulation technique thatuses pressurized liquid to fracture rock to allow trapped oil reservesto flow and be recovered. Diverting agents are used during hydraulicfracturing to temporarily plug existing fractures to allow foradditional pressurization and fracturing. The diverting agent must thenleave the fractures, typically by decomposing or dissolving, to optimizeoil extraction. The speed at which a diverting agent can degrade or beremoved matters. The U.S. Department of Energy reports that the cost tooperate an oil well can range from $250,000 to $450,000 per dayirrespective of well output because most of the cost relates toequipment and labor. Thus, any product that can speed oil extractionwill increase the profitability of the well.

Various compositions—from rock salt, oyster shells, and moth balls tomany kinds of degradable natural or synthetic polymers—have beenproposed as diverting agents.

Polylactic acid (“PLA”), a solid, degradable polymer made principally byring-opening polymerization of cyclic lactides, is commonly used as adiverting agent and may be the most commonly used diverting agent in thePermian Basin of southwestern Texas. The lower bottom hole temperaturewells of this fast-growing oil-and-gas exploration region range from 90°F. to 150° F., so diverting agents that can degrade rapidly atrelatively low temperature are needed. PLA does not degrade rapidly attemperatures below about 130° F. To overcome this problem, oil-and-gasprofessionals have resorted to mineral acids or other “degradationenhancers” that can help to accelerate decomposition of the divertingagent. Unfortunately, the acids or other degradation enhancers canpermanently damage well formations, so their usefulness is limited.

U.S. Pat. No. 9,580,642 describes “dissolvable diverters” that includephthalic anhydride, terephthalic acid, gilsonite, rock salt, benzoicacid flakes, and polymeric mixtures, among other diverters.Polylactides, especially those having weight-average molecular weightsfrom 100,000 to 200,000 g/mol are taught as preferred aliphaticpolyesters. In the examples, the reference teaches that PLA alone issuitable for use as a diverting agent at temperatures greater than 250°F., but combining it with a mixture of phthalic anhydride and phthalicacid can reduce the temperature at which PLA can dissolve to 180° F.

WO 2017/106,077 suggests using many of the same diverters taught in the'642 patent but in combination with a degradation enhancer, typically abasic compound such as an alkali metal hydroxide, an alkaline earthmetal carbonate, or an amine. U.S. Pat. No. 9,879,503 describes otherself-degradable diverting agents, including polyethylene glycol, citrateesters, glucose monoesters, partial fatty esters, PEG monolaurates, andtriacetin, among others. U.S. Pat. No. 8,109,335 recommends using adegradable fatty alcohol as one component of a diverting agent. U.S.Pat. No. 7,475,728 teaches to use an orthoester, poly(orthoester), or acombination thereof as a degradable diverting agent. No results arereported in these patents.

U.S. Pat. No. 9,657,557 teaches the use of polysaccharides incombination with polyester fibers, including PLA fibers, as divertingagents.

U.S. Pat. No. 9,090,810 describes various synthetic polymers suitablefor use as high-temperature diverters. Listed examples includepolyethylene terephthalate (PET), polybutylene succinate (PBS),polycaprolactone (PCL), polypropylene fumarate (PPF), andpolyhydroxyalkanoates (PHA).

The industry would benefit from the availability of diverting agentshaving the ability to quickly degrade at low temperature, particularlythe low temperatures characteristic of the lower bottom hole temperaturewells of the Permian Basin. Of interest are diverting agents that arecommercially available or can be easily synthesized by known methodsfrom inexpensive raw materials. Ideally, the diverting agents would beparticulate solids at the well temperature and would degrade moreeffectively than PLA without the need for a degradation enhancer attemperatures well below 150° F., or even below 130° F., to enhance theproductivity of the well.

SUMMARY OF THE INVENTION

In one aspect, the invention relates to a hydraulic fracturing methodfor recovering oil from a low-temperature subterranean oil formation.The method comprises: (a) before, during, or after inducing hydraulicfracturing within the formation, introducing into the formation aparticulate, degradable polyester diverting agent in an amount effectiveto improve oil production from the formation; (b) allowing the divertingagent to degrade; and (c) recovering oil from the formation. Thediverting agent has a melting point greater than the average temperaturewithin the formation. The diverting agent is selected from: (i) ethyleneglycol succinates; (ii) acid-terminated ethylene glycol succinates;(iii) acid-terminated polyglycolic acids; (iv) acid-terminatedpolylactic acids; (v) mixtures of any of (i) through (iv) with a halfacid ester; and (vi) mixtures of polylactic acids or polyglycolic acidswith a half acid ester.

We surprisingly found that certain classes of particulate polyestercompositions degrade more effectively under conditions present inlow-temperature wells, such as those found in the Permian Basin, thanpolylactic acid or other commonly used diverting agents. We also foundthat degradation times can be reduced by terminating polyester divertingagents with carboxylic acid groups or by combining them with a minorproportion of a half acid ester. The diverting agents are readilyavailable or are easily synthesized and should help to enhance theproductivity of low-temperature wells.

DETAILED DESCRIPTION OF THE INVENTION

In one aspect, the invention relates to a hydraulic fracturing methodfor recovering oil from a low-temperature subterranean oil formation.

By “low-temperature” subterranean oil formation, we mean a formationhaving an average temperature less than 180° F., typically within therange of 80° F. to 150° F., more typically from 110° F. to 130° F.

“Hydraulic fracturing” refers to a well-stimulation method in which afracturing fluid, typically water containing sand or other granularproppants, is injected under high pressure into a wellbore to createcracks in the formation that allow natural gas, oil, and brine to flowmore freely. Proppants help to keep the fractures open after hydraulicpressure is removed from the well.

“Diverting agent” refers herein to particulate materials that cantemporarily block or plug existing fractures and allow for additionalpressurization and fracturing. The diverting agent degrades, ideallywithin a week or two, thereby allowing petroleum products and/or brinesto flow and be recovered. The diverting agent is introduced into theformation before, during, or after inducing hydraulic fracturing withinthe formation.

The diverting agent is used in an amount effective to improve oilproduction from the formation.

Thus, in one aspect, the invention relates to a hydraulic fracturingmethod for recovering oil from a low-temperature subterranean oilformation. The method comprises: (a) before, during, or after inducinghydraulic fracturing within the formation, introducing into theformation a particulate, degradable polyester diverting agent in anamount effective to improve oil production from the formation; (b)allowing the diverting agent to degrade; and (c) recovering oil from theformation.

Diverting Agents

Suitable diverting agents are selected from (i) ethylene glycolsuccinates; (ii) acid-terminated ethylene glycol succinates; (iii)acid-terminated polyglycolic acids; (iv) acid-terminated polylacticacids; (v) mixtures of any of (i) through (iv) with a half acid ester;and (vi) mixtures of polylactic acids or polyglycolic acids with a halfacid ester.

1. Ethylene Glycol Succinates

In some aspects, the diverting agent is an ethylene glycol succinate,i.e., a polyester comprising recurring units of ethylene glycol andsuccinic acid (or succinic anhydride). Suitable ethylene glycolsuccinates can be made by condensation polymerization. In some aspects,the ethylene glycol succinates have primarily (or even exclusively)hydroxyl end groups. Such ethylene glycol succinates will have hydroxylvalues within the range of 14 to 400 mg KOH/g, or from 28 to 225 mgKOH/g, or from 40 mg to 112 mg KOH/g. In some aspects, the ethyleneglycol succinates will have number-average molecular weights within therange of 500 to 10,000 g/mol, or from 1,000 to 4,000 g/mol. In someaspects, the ethylene glycol succinate will have residual acidity fromcarboxylic acid end groups. Thus, in some aspects, the ethylene glycolsuccinates will have acid values within the range of 1 to 100 mg KOH/g,from 10 to 70 mg KOH/g, or from 20 to 50 mg KOH/g. For examples, seebelow at Table 1, Examples 1, 3, and 5, 6, and 8.

2. Acid-Terminated Ethylene Glycol Succinates

In some aspects, the diverting agent is an acid-terminated ethyleneglycol succinate. By “acid-terminated,” we mean a reaction product of anethylene glycol succinate with a cyclic anhydride or a dicarboxylicacid, preferably a cyclic anhydride, that generates a carboxylic acidend group from a hydroxyl end group. The ethylene glycol succinate isreacted with enough of the dicarboxylic acid or cyclic anhydride toacid-terminate some or all of the available hydroxyl groups. Suitablecyclic anhydrides or dicarboxylic acids are aliphatic (e.g., succinicanhydride, succinic acid, maleic anhydride, itaconic anhydride, maleicacid, fumaric acid) or aromatic (e.g., phthalic anhydride, trimelliticanhydride, pyromellitic dianhydride, isophthalic acid, terephthalicacid). Aromatic cyclic anhydrides are preferred.

In some aspects, an acid-terminated product is conveniently formed bycombining a freshly made ethylene glycol succinate while still warm witha desired proportion of a cyclic anhydride, preferably an aromaticcyclic anhydride. The cyclic anhydride ring readily opens to give theexpected half acid ester product. We found that the resulting acidityfrom phthalic acid residues from terminating ethylene glycol succinateswith phthalic anhydride can further promote degradation of the divertingagent (see Table 1, Examples 2 and 4, below).

The amount of cyclic anhydride (or dicarboxylic acid) used toacid-terminate the ethylene glycol succinate varies and depends thedesired acidity, degradability, solubility, melting range, the nature ofthe formation, well temperature, and other factors. Generally, however,the amount used will range from 0.1 to 20 wt. %, or from 1 to 15 wt. %,or from 5 to 10 wt. %, based on the combined amounts of ethylene glycolsuccinate and cyclic anhydride or dicarboxylic acid used.

3. Acid-Terminated Polyglycolic Acids

In some aspects, the diverting agent is an acid-terminated polyglycolicacid. By “acid-terminated,” we mean a reaction product of a polyglycolicacid with a cyclic anhydride or a dicarboxylic acid, preferably a cyclicanhydride, that generates a carboxylic acid end group from a hydroxylend group. Suitable cyclic anhydrides or dicarboxylic acids aredescribed in Section 2.

Polyglycolic acid (“PGA” or “polyglycolide”) is known as a degradablediverting agent. Although some literature suggests that PGA havingrelatively high weight-average molecular weight (100,000 to 200,000g/mol) and produced by ring-opening polymerization should be used, atleast one product having a molecular weight of about 600 g/mol wascommercially available from DuPont in 2010 (Polyglycolic acid TLF 6267).The high-molecular-weight product, which is used for dissolvablesutures, is expensive. However, desirable PGA of low number-averagemolecular weight are conveniently made by dehydrating 70% aqueousmixtures of glycolic acid, which are inexpensive and readily available.We found that the low-molecular-weight PGA (desirably, number-averagemolecular weights of 400 to 5,000 g/mol or 500 to 2,000 g/mol) degradesreadily at temperatures below 150° F., and even at 110-120° F. However,the degradation rate can be further increased by reacting the PGA with aminor proportion of a cyclic anhydride (e.g., phthalic anhydride) or adicarboxylic acid is used to acid-terminate some or all of the hydroxylend groups (see Table 1, Examples 10 and 12 and Comparative Examples 9and 11, below).

The amount of cyclic anhydride (or dicarboxylic acid) used toacid-terminate the PGA varies and depends the desired acidity,degradability, solubility, melting range, the nature of the formation,well temperature, and other factors. Generally, however, the amount usedwill range from 0.1 to 20 wt. %, or from 1 to 15 wt. %, or from 5 to 10wt. %, based on the combined amounts of PGA and cyclic anhydride ordicarboxylic acid used.

4. Acid-Terminated Polylactic Acids

In some aspects, the diverting agent is an acid-terminated polylacticacid. By “acid-terminated,” we mean a reaction product of a polylacticacid with a cyclic anhydride or a dicarboxylic acid, preferably a cyclicanhydride, that generates a carboxylic acid end group from a hydroxylend group. Suitable cyclic anhydrides or dicarboxylic acids aredescribed in Section 2.

Polylactic acids (“PLA” or “polylactides”) are well-known divertingagents and are commercially available (e.g., BIOVERT® NWB polylactidefrom Halliburton). Although polylactic acids degrade effectively athigher temperatures (e.g., 150° F.), we found that they degrade muchless effectively at 120° F., a temperature that is common in many wellsin the Permian Basin of southwestern Texas. As noted above, we foundthat acid termination of PGA with a cyclic anhydride or dicarboxylicacid improves the degradability of the PGA, and we expect thatdegradability of the structurally similar PLA materials to improve withacid termination.

The amount of cyclic anhydride or dicarboxylic acid used toacid-terminate the PLA varies and depends the desired acidity,degradability, solubility, melting range, the nature of the formation,well temperature, and other factors. Generally, however, the amount usedwill range from 0.1 to 20 wt. %, or from 1 to 15 wt. %, or from 5 to 10wt. %, based on the combined amounts of PLA and cyclic anhydride ordicarboxylic acid used.

5. Mixtures Containing a Half Acid Ester in Some Aspects, the DivertingAgent, which can be any of the Materials Described in the precedingsections 1-4, is combined with a minor proportion of a half acid ester(herein also described as “HAE”). The combination provides a furtherimprovement in degradability.

As used in this application, “half acid ester” refers to a reactionproduct of an alcohol or a polyol with a cyclic anhydride or adicarboxylic acid. The reaction is performed under conditions effectiveto minimize or avoid oligomerization or condensation polymerization andto produce principally a 1:1 adduct of the alcohol or polyol and thecyclic anhydride or dicarboxylic acid. Reaction temperatures aretypically less than 140° C., less than 130° C., or less than 120° C. Theresulting half acid ester product has at least one, preferably one, freecarboxylic acid group. An example is the half acid ester made byreacting phthalic anhydride with diethylene glycol at temperatures belowabout 120° C.:

In some aspects, the HAE is made by reacting an alcohol or a polyol,usually an aliphatic diol, with an equimolar amount of a cyclicanhydride or a dicarboxylic acid, preferably a cyclic anhydride.Suitable alcohols or polyols have one or more free hydroxyl groups, andinclude, for example, methanol, ethanol, butanols, 2-ethylhexanols,cyclohexanol, benzyl alcohol, ethylene glycol, propylene glycol,diethylene glycol, dipropylene glycol, triethylene glycol, tripropyleneglycol, 1,3-propanediol, 2-methyl-1,3-propanediol, 1,2-butylene glycol,1,3-butylene glycol, 1,4-butanediol, neopentyl glycol, 1,6-hexanediol,cyclohexanediols, glycerol, trimethylolpropane, trimethylolethane, andthe like, and mixtures thereof. In some aspects, the alcohol or polyolis an alkylene glycol, e.g., ethylene glycol, propylene glycol,diethylene glycol, or the like. Diethylene glycol is a preferredalkylene glycol.

The cyclic anhydride or dicarboxylic acid and the alcohol or polyol canbe combined and reacted under any suitable conditions effective to formthe half acid ester. Usually, mild heating is enough to form the desired1:1 molar reaction product and minimize further esterification fromcondensation.

The amount of HAE combined with the diverting agent varies and dependsthe desired acidity, degradability, solubility, melting range, thenature of the formation, well temperature, and other factors. Generally,however, the amount of HAE used will range from 0.1 to 20 wt. %, or from1 to 15 wt. %, or from 5 to 10 wt. %, based on the combined amounts ofHAE and diverting agent used.

We found, for instance, that the degradability of an ethylene glycolsuccinate could be improved significantly by combining it with 5 wt. %of an HAE made from diethylene glycol and phthalic anhydride (in Table1, compare Example 7 with Example 6).

6. Mixtures of PLA or PGA with a Half Acid Ester

Similarly, the HAEs described above can be used in combination with PLA,PGA, or mixtures thereof to effect an improvement in degradability ofthe PLA or PGA diverting agent.

The amount of HAE combined with the PLA or PGA diverting agent variesand depends the desired acidity, degradability, solubility, meltingrange, the nature of the formation, well temperature, and other factors.Generally, however, the amount of HAE used will range from 0.1 to 20 wt.%, or from 1 to 15 wt. %, or from 5 to 10 wt. %, based on the combinedamounts of HAE and PGA or PLA (or mixture thereof) diverting agent used.

We found, for instance, that the degradability of an PGA diverting agentcould be improved significantly by combining it with 5 wt. % of an HAEmade from diethylene glycol and phthalic anhydride (see Table 1, Example13 and Comparative Example 11).

The diverting agents used herein are particulate materials that havemelting points greater than the average temperature of the formationsuch that the diverting agent can be introduced into the formationwithout a significant degree of melting. In some aspects, the divertingagents have melting points above room temperature, i.e., greater than70° F., and usually greater than 100° F. Ideally, the diverting agentcan be pulverized or ground, then sieved to a desired particle sizerange. In some cases, it may be desirable to combine particles havingvery different particle size distributions.

Diverting agents can be introduced into the formation by any desiredmeans, including, for example, injection “as is” as solid particles,injection as a mixture of a gas (e.g., air or nitrogen) and solidparticles, injection as a suspension or emulsion of particles in water,a brine, or a hydrocarbon, injection as a foam or foamable mixture thatcontains the solid particles, injection as a solid mixture with sand oranother proppant or other additive, or the like. Most conveniently, thediverting agent is introduced into the formation as a dilute aqueoussuspension that may also include a proppant.

The amount of diverting agent used will be an amount effective toimprove oil production from the formation. The exact amount requiredwill depend on many factors, including the well location, welltemperature, age and degree of development of the well, nature of theformation, the particular diverting agent used, and other factors, andis left to the skilled person's discretion. In general, diverting agentsare dosed to a well in an amount within the range of 1 ppm to 30 wt. %,or from 100 ppm to 10 wt. %, or from 1,000 ppm to 1 wt. %, based on theamount of charged mixture that contains the diverting agent.

The diverting agent is allowed to remain in contact with the formationlong enough for the diverting agent to degrade. The time needed forsignificant and helpful degradation to occur depends on the nature ofthe formation, well temperature, salinity, acidity of the formation,whether an acid/base degradation enhancer is used, the nature and amountof the diverting agent, productivity requirements, and other factors.Generally, significant degradation of the diverting agent occurs within14 days, preferably within 7 days, within 3 days, or in some cases,within 24 hours. In some aspects, the diverting agent can degrade withinthe formation at temperatures less than 150° F. to lose at least 10%,20%, 30%, or 50% of its mass within 14 days. In other aspects, thediverting agent can degrade within the formation at temperatures lessthan 130° F. to lose at least 10%, 20%, 30%, or 50% of its mass within14 days.

Oil is recovered from the formation by conventional means. Degradationof the diverting agent allows oil liberated by fracturing to flowthrough passages that were blocked temporarily or in part by particlesof diverting agent and/or proppants. As the diverting agent degrades,the passages become more open and oil can flow to a production well orother oil recovery zone.

In principle, the diverting agents described herein can be used in wellscharacterized by relatively low or relatively high temperatures. Anadvantage of these diverting agents is their ability to degradeeffectively at even low temperatures. In some aspects, the formationwill have an average temperature less than 180° F., or within the rangeof 90° F. to 150° F., or from 100° F. to 130° F.

The following examples merely illustrate the invention; the skilledperson will recognize many variations that are within the spirit of theinvention and scope of the claims.

Commercial Materials:

The hydroxyl values and acid values indicated below are typical values.

BIOVERT® polylactide (Halliburton): lactic acid homopolymer divertingagent.

STEPANPOL® PC-101P-56 (Stepan): aliphatic polyester polyol from ethyleneglycol and adipic acid. OH value: 56 mg KOH/g.

STEPANPOL® PC-102-56 (Stepan): aliphatic polyester polyol from1,4-butanediol and adipic acid. OH value: 56 mg KOH/g.

RUCOTE® XP 5500 (Stepan): acid-functional polyester. Acid value: 75 mgKOH/g;

OH value: <5 mg KOH/g.

RUCOTE® 562 (Stepan): acid-functional polyester. Acid value: 55 mgKOH/g; OH value: 8 mg KOH/g.

RUCOTE® 9006 (Stepan): acid-functional polyester. Acid value: 36 mgKOH/g; OH value: 4 mg KOH/g.

Cheesecloth Test

Hydrolysis or decomposition of candidate materials is evaluated asfollows. A 3″×3″ square of triple-layered, retail-grade cheesecloth (andcheesecloth strip used as a tie) are cut and weighed. Polymer testsample (about 5 g) is added to the center of the cheesecloth, and theends are brought together and tied with the strip to enclose the sample.The cheesecloth is added to a heavy-gauge PYREX® pressure test tube witha TEFLON® seal along with deionized water (100 mL). Sealed tubes areplaced in separate ovens maintained at 110° F., 120° F. and 150° F. Thecheesecloth assemblies are removed from the tubes after 4, 7 and 14days, patted dry, and further dried to constant weight in a 90° C.forced-air oven. Dry assembly weights are then recorded to allowcalculation of weight percent loss for each sample.

Polyglycolic Acid (Comparative Product)

Aqueous glycolic acid (331 g of 70 wt. % solution) is added to areaction flask equipped with a nitrogen sparge, agitator, thermocouple,temperature controller, heating mantle, and condenser. The mixture isheated to 130° C. to allow the water initially supplied with theglycolic acid to evaporate. Additional glycolic acid solution (102 g) isthereafter introduced dropwise. Once the water removal slows, thereactor contents are heated to 220° C. Titanium(IV) butoxide (one drop,about 20 mg) is added, and the mixture is allowed to react for 4-6 hourswith a nitrogen sweep at 220° C. The reaction product has a meltingpoint (by differential scanning calorimetry at 10° C./min) of 191° C.The hot liquid polyglycolic acid (“PGA”) product is transferred to asuitable container and cools to room temperature. Samples are groundwith mortar and pestle for the cheesecloth test. Samples taken from thereactor at different reaction times are labeled “PGA(1)” and “PGA(2)”(see Table A and Table 1).

Polyglycolic Acid with Phthalic Anhydride Cap (Inventive Product)

The apparatus described above is charged with aqueous glycolic acid (488g of 70% solution). The mixture is heated to 130° C. to allow the waterinitially supplied with the glycolic acid to evaporate, and the reactiontemperature is increased to 190° C. Additional glycolic acid solution(173 g) is thereafter introduced dropwise. Once the water removal slows,the reactor contents are heated to 220° C. Titanium(IV) butoxide (onedrop, about 20 mg) is added, and the mixture is allowed to react for 4-6hours with a nitrogen sweep at 220° C. A 20-g sample of the finalproduct is removed as a control. Next, phthalic anhydride (158 g) isadded to the remaining hot liquid polyglycolic acid (300 g), and themixture reacts for 10 min. Product samples are cooled and ground withmortar and pestle for the cheesecloth test.

Ethylene Glycol Succinate with High Acid Values

The above-described apparatus is modified to include a 10″stainless-steel packed column between the reactor and the condenser.Succinic acid (273.5 g) and ethylene glycol (156.5 g) are added to thereactor. The contents are agitated under a nitrogen sparge and heated to220° C. to remove water formed in the condensation reaction. When waterremoval is complete, titanium(IV) butoxide (0.04 g, 93 ppm) is added.Ten-mL samples are collected at various reaction stages; the sampleshave acid values of 52, 38, 32 and 21 mg KOH/g.

Ethylene Glycol Succinate Terminated with Phthalic Anhydride

The apparatus described immediately above is charged with succinic acid(580 g) and ethylene glycol (319 g). Titanium(IV) butoxide catalyst isadded as before. When the product acid value reaches 1 mg KOH/g, asample of the product (70.6 g) is removed and retained as a control;this sample has a hydroxyl value of 20 mg KOH/g. Another sample (70.7 g)is removed and combined with phthalic anhydride (3.7 g) to form thecorresponding half acid ester. Ethylene glycol (9.4 g) is added to theremaining reactor material to raise the hydroxyl value to 46 mg KOH/g,and a sample (70.3 g) of this product is removed. To this sample,phthalic anhydride (8.5 g) is added to make a half acid ester. All foursamples are then tested using the cheesecloth test at 120° F. The 46hydroxyl value samples, both with and without phthalic anhydridemodification, are also tested at 110° F. and 150° F.

Half Acid Ester from Phthalic Anhydride and Diethylene Glycol

A 1:1 molar reaction product of phthalic anhydride and diethylene glycol(CAS 2202-98-4, 2-(2-hydroxyethoxy)ethyl hydrogen phthalate, hereinaftersimply “half acid ester” or “HAE”) is prepared by briefly mixingphthalic anhydride (148 g) with diethylene glycol (106 g) at about 250°F. (120° C.) in a beaker until a clear solution is obtained. The HAE isblended as needed with hot polyester polyols.

Melting Point Determination

Product thermal transitions (melting points) of various products arecharacterized by a TA Instruments Discovery differential scanningcalorimeter (DSC) with Trios analysis software by heating from 25° C. to200° C. at 10° C./min.

Acid values: ASTM D4662-15Hydroxyl values: ASTM E-222-17

The hydroxyl values and number-average molecular weights for the PGAsamples (Examples C9-13) are determined by ¹H NMR spectroscopy inpyridine-d5.

Results:

Table A lists physical properties of the polyester materials prepared ortested. Table 1 summarizes results of the cheesecloth test used toevaluate polyol decomposition (i.e., mass loss) as a function of time ata particular temperature.

BIOVERT® polylactide, a commercial diverting agent, decomposes rapidlyat 150° F., but its ability to decompose at 120° F. is unsatisfactory(less than 2% after 7 days) as shown in Comparative Example 14.Consequently, the polylactide would be less suitable for use in alow-temperature formation such as those found in the Permian Basin insouthwestern Texas.

Other commercially available polyester polyols were also found to givepoor decomposition rates in the cheesecloth test. For instance, severalRUCOTE® polyesters (acid-terminated polyesters) failed to decomposesignificantly after 4 days at 150° C. (Comparative Examples 15-17), andtwo adipate polyester polyols, STEPANPOL® PC-101P-56 and STEPANPOL®PC-102-56, failed to decompose significantly after 4 days at 120° F.(Comparative Examples 18 and 19).

We surprisingly found that ethylene glycol succinate, which has alimiting ester content equivalent to that of polylactic acid (each atabout 61 wt. %), degrades more effectively in the cheesecloth test thanthe commercial polylactide when tested at 120° F. (see Examples 1 and3). Degradation of the ethylene glycol succinate could be improved byusing products having higher acid values (Examples 5, 6, and 8).

Additional improvements in degradability of the ethylene glycolsuccinates are achieved by acid-terminating the polyesters with phthalicanhydride (Examples 2 and 4).

Interestingly, a simple admixture of the ethylene glycol succinate witha half acid ester (HAE) produced by reacting phthalic anhydride with onemolar equivalent of diethylene glycol improves degradability (compareExample 7 with Example 6).

Polyglycolic acid (PGA) is another known diverting agent.High-molecular-weight PGA materials made by ring-opening polymerizationare relatively expensive. Low-molecular-weight PGA (e.g., 1000 g/mol orless) can be made conveniently and cost-effectively by dehydration of a70% aqueous glycolic acid solution, and similar materials have beensuggested previously for use as diverting agents. In our experiments,low-molecular-weight PGA degrades well at 120° F. (Comparative Examples9 and 11). However, the degradability of low-molecular-weight PGA can befurther enhanced by acid-terminating with phthalic anhydride (Examples10 and 12) or by combining the PGA with a minor proportion (e.g., 5 wt.%) of the HAE material described above.

TABLE A Polyester Properties hydroxyl acid Number-avg. value valuemolecular (mg (mg weight Ex KOH/g) KOH/g) (g/mol)  1 ES, 46 OH 47.2 1.02330  2 ES, 46 OH + PAn 34.6 12 2410  3 ES, 20 OH 21.0 1.0 5110  4 ES,20 OH + PAn 15.8 6.0 5170  5 ES, 21 AV 56.0 21 1460  6 ES, 32 AV 56.0 321280  7 ES, 32 AV + 5% HAE 53.2 41 1000  8 ES, 52 AV 56.0 52 1040 C9PGA(1) 103 103 545 10 PGA(1) + PAn 0 309 693 C11 PGA(2) 47.6 47.6 118012 PGA(2) + PAn 41.5 54.9 1200 13 PGA(2) + 5% HAE 46.3 46.3 1130 C14BIOVERT ® polylactide − − − C15 RUCOTE ® XP 5500 <5.0 75 1400 C16RUCOTE ® 562 8.0 55 1780 C17 RUCOTE ® 9006 4.0 36 2810 C18 STEPANPOL ®PC-101P-56 56.3 0.12 1990 C19 STEPANPOL ® PC-102-56 58.0 0.33 1920 ES =ethylene glycol succinate; PGA = polyglycolic acid; PAn = phthalicanhydride reactant; HAE = half ester of PAn and diethylene glycoladditive at 5 wt. %. OH = hydroxyl value in mg KOH/g; AV = acid value inmg KOH/g. BIOVERT ® polylactide diverting agent, product of Halliburton.RUCOTE ® acid-terminated polyesters and STEPANPOL ® polyester polyolsare products of Stepan Company.

TABLE 1 Polyester Polyol Decomposition Results, Cheesecloth Test (% massloss versus time) 110° F. 120° F. 150° F. Ex 4 days 7 days 14 days 4days 7 days 14 days 4 days 7 days 14 days 1 ES, 46 OH 7.4 8.6 9.2 7.08.2 8.8 11.4 12.4 19.8 2 ES, 46 OH + PAn 16.2 18.2 20.2 17.7 20.0 24.239.4 41.6 58.0 3 ES, 20 OH 4.2 5.2 5.4 4 ES, 20 OH + PAn 9.2 9.8 11.4 5ES, 21 AV 8.2 13.8 18.6 6 ES, 32 AV 7.4 17.7 22.7 7 ES, 32 AV + 5% HAE12.2 24.7 26.6 8 ES, 52 AV 7.6 28.1 39.5 C9  PGA(1) 24.7 33.3 37.5 33.439.8 43.4 37.0 61.2 77.3 10 PGA(1) + PAn 28.5 38.9 42.5 41.6 44.7 54.252.6 69.0 82.6 C11 PGA(2) 18.6 29.2 39.1 12 PGA(2) + PAn 23.4 37.7 50.513 PGA(2) + 5% HAE 34.1 40.5 51.1 C14 BIOVERT ® polylactide 2.0 1.7 —29.5 34.9 52.5 C15 RUCOTE ® XP 5500 <1 — — C16 RUCOTE ® 562 <1 — — C17RUCOTE ® 9006 <1 — — C18 STEPANPOL ® PC-101P-56 <1 — — C19 STEPANPOL ®PC-102-56 <1 — — ES = ethylene glycol succinate; PGA = polyglycolicacid; PAn = phthalic anhydride reactant; HAE = half ester of PAn anddiethylene glycol additive at 5 wt. %. OH = hydroxyl value in mg KOH/g;AV = acid value in mg KOH/g. BIOVERT ® polylactide diverting agent,product of Halliburton. RUCOTE ® acid-terminated polyesters andSTEPANPOL ® polyester polyols are products of Stepan Company.

The preceding examples are meant only as illustrations; the followingclaims define the inventive subject matter.

1. A hydraulic fracturing method for recovering oil from alow-temperature subterranean oil formation, the method comprising: (a)before, during, or after inducing hydraulic fracturing within theformation, introducing into the formation a particulate, degradablepolyester diverting agent in an amount effective to improve oilproduction from the formation; (b) allowing the diverting agent todegrade; and (c) recovering oil from the formation; wherein thediverting agent has a melting point greater than the average temperaturewithin the formation; and wherein the diverting agent is selected fromthe group consisting of: (i) ethylene glycol succinates; (ii)acid-terminated ethylene glycol succinates; (iii) acid-terminatedpolyglycolic acids; (iv) acid-terminated polylactic acids; (v) mixturesof any of (i) through (iv) with a half acid ester; and (vi) mixtures ofpolylactic acids or polyglycolic acids with a half acid ester.
 2. Themethod of claim 1 wherein the diverting agent is an ethylene glycolsuccinate having an acid value within the range of 1 mg KOH/g to 100 mgKOH/g.
 3. The method of claim 1 wherein the diverting agent is anethylene glycol succinate having an acid value within the range of 10 mgKOH/g to 70 mg KOH/g.
 4. The method of claim 1 wherein the divertingagent is an ethylene glycol succinate having a hydroxyl value within therange of 14 mg KOH/g to 400 mg KOH/g.
 5. The method of claim 1 whereinthe diverting agent is mixture of the ethylene glycol succinate and 1 to15 wt. %, based on the amount of diverting agent, of a half acid ester.6. The method of claim 5 wherein the half acid ester is a reactionproduct of phthalic anhydride and diethylene glycol.
 7. The method ofclaim 1 wherein the diverting agent is a mixture of polyglycolic acidhaving a number-average molecular weight within the range of 400 to5,000 g/mol and 1 to 15 wt. %, based on the amount of diverting agent,of a half acid ester.
 8. The method of claim 7 wherein the half acidester is a reaction product of phthalic anhydride and diethylene glycol.9. The method of claim 1 wherein the diverting agent is anacid-terminated polyglycolic acid made from a cyclic anhydride andpolyglycolic acid having a number-average molecular weight within therange of 400 to 5,000 g/mol.
 10. The method of claim 1 wherein thediverting agent is an acid-terminated ethylene glycol succinate made byreacting an ethylene glycol succinate with a cyclic anhydride.
 11. Themethod of claim 10 wherein the cyclic anhydride is phthalic anhydride.12. The method of claim 1 wherein the formation has an averagetemperature less than 180° F.
 13. The method of claim 1 wherein theformation has an average temperature within the range of 90° F. to 150°F.
 14. The method of claim 1 wherein the formation has an averagetemperature within the range of 100° F. to 130° F.
 15. The method ofclaim 1 wherein the diverting agent has a melting point at least 30Fahrenheit degrees greater than the average temperature of theformation.
 16. The method of claim 1 wherein the diverting agent candegrade within the formation at temperatures less than 150° F. to loseat least 10% of its mass within 14 days.
 17. The method of claim 1wherein the diverting agent can degrade within the formation attemperatures less than 130° F. to lose at least 20% of its mass within14 days.